Class SecretKeyFactorySpi

This class defines the Service Provider Interface (SPI)
for the SecretKeyFactory class.
All the abstract methods in this class must be implemented by each
cryptographic service provider who wishes to supply the implementation
of a secret-key factory for a particular algorithm.

A provider should document all the key specifications supported by its
secret key factory.
For example, the DES secret-key factory supplied by the "SunJCE" provider
supports DESKeySpec as a transparent representation of DES
keys, and that provider's secret-key factory for Triple DES keys supports
DESedeKeySpec as a transparent representation of Triple DES
keys.

engineGetKeySpec

Returns a specification (key material) of the given key
object in the requested format.

Parameters:

key - the key

keySpec - the requested format in which the key material shall be
returned

Returns:

the underlying key specification (key material) in the
requested format

Throws:

InvalidKeySpecException - if the requested key specification is
inappropriate for the given key (e.g., the algorithms associated with
key and keySpec do not match, or
key references a key on a cryptographic hardware device
whereas keySpec is the specification of a software-based
key), or the given key cannot be dealt with
(e.g., the given key has an algorithm or format not supported by this
secret-key factory).